This invention relates to the field of automatic testing of printed circuit boards, and more particularly, it relates to a fixturing system which can be used with automatic test equipment.
In order to test an electrical product such as a printed circuit board, means must be provided for connecting the circuit board to the test system. Where in-circuit tests are to be conducted, a plurality of connections must be made to some or all of the nodal points of the circuits being tested. This is commonly done by means of probes which are individually connected to the test system and which can be brought into contact with the various nodal points in the circuit paths on the backs of the circuit boards being tested. The device containing the probes is often referred to as a fixture or a product access unit.
One popular type of a fixture used in in-circuit testing today utilizes a vacuum both to draw the circuit board being tested down tightly on the fixture and to cause the probes to advance into contact therewith. The probes are typically spring loaded on a platen which is drawn upwardly as the interior of the fixture is evacuated until the probes make contact with the circuit board. These probes are normally connected by wires to a plugboard which, in turn, is connected to the test system. Generally, the vacuum chamber portion of the fixture is held vertically above the plugboard by metal braces. It can be appreciated that neither the underside of the probes contained in the upper vacuum chamber portion of fixtures such as this, nor the connections to the plugboard are very accessible due to the physical arrangements just described and the presence of a plurality of wires running down from the probes to the plugboard. This is unfortunate because there are times when it is advantageous to have access to individual electrical connections contained in these areas. One may wish, for example, to check a wiring connection or to attach an electrical measuring instrument of some type to a probe in order to monitor that probe as the test system is operated. Another disadvantage associated with the kinds of fixtures just described is that they are quite expensive to manufacture. The top plates of these fixtures, for example, must be precision drilled in order to provide holes to permit the probes to pass therethrough, and at the same time, to serve as guides in maintaining the alignment of the probes. Also, the configuration of this kind of a fixture does not lend itself to the use of cost-saving wire-wrapping tools. Yet another disadvantage associated with this type of fixture concerns storage. Because the vacuum chamber and plugboard are held in a spaced-apart relationship by metal struts, they necessarily occupy a relatively large volume of space. This can be quite significant because most users of automatic test equipment test a great number of circuit boards of various sizes and complexity. Since, in general, a separate fixture is required for each type of board to be tested, a substantial number of fixtures must be stored. Yet another disadvantage of the type of fixture here under discussion is that it is relatively cumbersome to handle and to attach to the receiver of an automatic test system. The standard plugboard arrangements, with exposed wires, although adequate for this application, are far from the optimum in terms of ease of handling and avoidance of unnecessary damage to the fixture and receiver. Also, the operator time consumed in changing fixtures is longer than it need be. A final disadvantage common to the kind of fixture here under discussion concerns the seal between the movable platen carrying the test probes and the housing of the vacuum chamber. This seal is normally made by gluing a strip of gasket material around the border of the platen so that it overhangs the edge of the platen and touches the inside of the housing. Since the housing is usually a metal casting, its interior surface is relatively rough and a significant amount of leakage normally occurs. Also, the scraping of the gasket against this housing as the platen moves up and down tends to cause wearing of the gasket over a period of time and even greater leakage. Depending on usage, the efficiency of the seal will diminish to the point where repair is required. All of the above disadvantages are eliminated by the present invention by the provision of a novel product access unit as well as a novel receiver and means for aligning and assembling the product access unit with the receiver. These elements are brought together in a systemic approach permitting maximum utilization of cost-saving techniques in their fabrication. Means are also provided for using these novel structures with existing test systems having conventional plugboard receivers.